Enable input methods for QQuickWidget

Make it possible for widgets to have non-widget focus objects. Since
we cannot add new virtual functions due to binary compatibility, we
have to do it in QWidgetPrivate.

Task-number: QTBUG-42677
Change-Id: I5f74daed2793c1c149bbe02e54ff2f7e2ad1af9e
Reviewed-by: Jørgen Lind <jorgen.lind@theqtcompany.com>

src/widgets/kernel/qwidget_p.h

@@ -619,6 +619,8 @@ public:
 
     QOpenGLContext *shareContext() const;
 
+    virtual QObject *focusObject() { return 0; }
+
 #ifndef QT_NO_OPENGL
     virtual GLuint textureId() const { return 0; }
     virtual QImage grabFramebuffer() { return QImage(); }

src/widgets/kernel/qwidgetwindow.cpp

@@ -147,6 +147,12 @@ QObject *QWidgetWindow::focusObject() const
     if (!widget)
         widget = m_widget;
 
+    if (widget) {
+        QObject *focusObj = QWidgetPrivate::get(widget)->focusObject();
+        if (focusObj)
+            return focusObj;
+    }
+
     return widget;
 }